Symptoms: Today it happened that I had a 100% of some torrent in tixati BT client, sharing it via the I2P network only (no clearnet). I was announcing myself to bunch of BT trackers. There were buch of other BT peers having 0% of the same torrent, they were the I2PSnark clients with default configuration. So far so good, BUT tixati was unable to share the content with other I2PSnarks.

Treatment attempt: I know this because I was the one running the other I2PSnarks. I obtained the I2PSnark's destinations (IP address equivalent) and manually added them to tixati. Still, no sharing occurred, tixati said that the other side closed the connection.

Cause: I2PSnark uses the i2cp.leaseSetEncType=6,4 configuration by default, tixati uses the i2cp.leaseSetEncType=4,0 configuration by default. They are not compatible with each other.

Cure: I went to all my I2PSnarks and added the i2cp.leaseSetEncType=4,0 configuration to them. This fixed the issue, all the Snarks started to download the content from my tixati.

Future: Please implement post quantum crypto for the I2P networking in tixati. More specifically public key type 6 aka MLKEM768_X25519. This will improve sharing inside the I2P network. I2PSnark is the default BT client for majority of I2P users, their number will only increase in future.

I just tested with I2P v2.10.0 using the built-in I2PSnark client.  I could connect to tixati seeds no problem.  If you saw any difference it may have been coincidence.  I2PSnark is quite slow and sometimes takes a few minutes to even try to connect to seeds.

In I2PSnark config, i2cp option i2cp.leaseSetEncType indicates to the client which algos should be used to setup the lease encryption keys.  The parameters "6,4" mean it supports the newer post-q (6) AND the well-established x25519 (4).  If you change it to "4,0" it means x25519 (4) and the old el-gamal (0).  In both instances, you are publishing x25519, which virtually all I2P routers support.  This wouldn't have made any difference.

https://geti2p.net/en/docs/protocol/i2cp#options

I2PSnark and Tixati both use X25519.  That's what the 4 is in leaseSetEncType.  They connect just fine.  Adding MLKEM768_X25519 won't make any difference at all until X25519 is retired, which is many years away.